More efficient EvmPooledMemory

[benaadams]

Changes

• Reduce conversions int -> UInt256
• Single reg256 copy for SaveWord rather than invoking Span.CopyTo

Types of changes

What types of changes does your code introduce?

• Bugfix (a non-breaking change that fixes an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (a change that causes existing functionality not to work as expected)
• Optimization
• Refactoring
• Documentation update
• Build-related changes
• Other: Description

Testing

Requires testing

• Yes
• No

If yes, did you write tests?

• Yes
• No